This invention generally relates to the field of radio communication systems and, more particularly, to increasing channel allocation efficiency in time division multiple access (TDMA) communication systems that utilize time division duplex (TDD) or frequency division duplex (FDD) access.
A TDMA communication system subdivides one or more radio frequency (RF) channels into a number of time slots during which mobile stations within the system exchanges voice or data and control information with each other. Generally, in these system voice or data information are communicated over time slots known as traffic channels and control information are communicated over time slots known as control channels. Through a plurality of scattered base stations, the mobile stations engage in duplex communication by transmitting bursts of digital information during allocated uplink and downlink time slots. An uplink time slot is a time slot allocated for transmitting the bursts from the mobile station to the base station, and a downlink time slot is a time slot allocated for transmitting the bursts from the base station to the mobile station.
Existing TDMA systems utilize a number of access techniques that support duplex communication. Two well known techniques include frequency division duplex (FDD) access and time division duplex (TDD) access. A TDMA/FDD communication system, such as one based on the Global System for Mobile (GSM) communication standard, uses separate receive and transmit RF channels, which are subdivided into respective uplink and downlink time slots. Another TDMA/FDD system, a digital system known as D-AMPS, which is based on Telecommunication Industry Association (TIA) IS 136 standard. The mobile stations in the GSM system transmit information on the transmit RF channels and receive information on the receive RF channels. The receive RF channel has a fixed frequency offset from the transmit RF channel. Based on allocated RF channels, a local controller in the mobile station determines the receive and transmit RF channels in accordance with the fixed frequency offset.
In contrast, a TDMA/TDD communication system provides duplex communications using a single frequency channel that is divided into a plurality of uplink and downlink time slots. One type of TDMA/TDD communication system is based on the Digital European Cordless Telecommunications (DECT) standard, which defines a protocol that can support high traffic as required for many business applications, such as office environments. Under the DECT each other through a decentralized dynamic channel allocation procedure, which allows various system providers to offer communication services without acquiring specific frequency channel allocation from governmental agencies. Another TDMA/TDD system is the PHS system, which is a DECT like system for the pacific market.
In both the TDD and FDD systems, a geographical area is divided into a number of cells or micro-cells according to a predefined frequency reuse cell pattern. The larger communication cells usually cover areas within which fast moving mobile stations operate, for example, those operating from a moving automobile. The smaller micro-cells cover areas where slow moving mobile stations operate, for example, those operated by subscribers within a small office complex or a building. The base stations, which cover corresponding communication cells or micro-cells, communicate with the mobile stations over the RF channels that are divided into uplink and downlink time slots. In response to call requests to and from the mobile stations, the base stations allocate particular uplink and downlink time slots during which corresponding mobile stations can engage in duplex communication with each other or with terminals of a private or public network, such as a public switched telephone network (PSTN).
Often, a calling mobile station may want to place a call to a called mobile station located in close proximity, for example, in the same communication cell as the calling mobile station. Conventionally, in this situation, the communication system establishes a voice path from each mobile station to the base station. As a result, the communication system must allocate a set of uplink and downlink time slots to each one of the calling and the called mobile stations. For example, in GSM systems, the base station allocates to the calling and the called mobile stations one uplink and one downlink time slots, and in D-AMPS systems, it allocates two uplink and two downlink time slots. Conventionally, to place a call between two mobile stations, an uplink and a downlink channel to the base station is allocated, resulting in the use of four links for handling each intra-cell call. Therefore, when handling intra-cell calls in congested office environments according to the conventional method, communication throughput quickly diminishes as the number of intra-cell calls increases.
One solution to this problem lies in allowing the mobile stations to communicate directly with each other. By communicating directly, the mobile stations use only one half of the time slots compared to indirect communication. Subsequently, one of the mobile stations transmits burst of information over a digital traffic channel, while the other receives them. During a second time slot, the mobile stations transmit and receive messages in an opposite order in time, thereby reducing the number of allocated time slots by half. An Annex to the DECT standard, Annex G, proposes a direct communication mode. The direct mode is activated via a mode switch positioned on the mobile stations. Because the direct mode is activated under the control of the subscribers, consequently, the base stations do not participate in the setting up and handling of the direct calls. There is, therefore, a high probability that the users, who generally have little or no knowledge of channel allocation within the system, would not engage in direct communication in a manner that would improve channel allocation efficiency. Furthermore, direct calls according to the Annex are initiated without any knowledge of the distance between calling and called mobile stations, thereby increasing the chance of an unsuccessful call set-up.
Therefore, channel availability may remain constrained, despite the proposed direct communication capability between the mobile stations.
There are communication systems based on FDD access techniques that allow direct communication between mobile stations. One such system, known as a trunked system, can centrally control direct communication between the mobile stations. There exist both analog and digital trunked systems. In this type of system, a central controller, in response to requests for direct communication, allocates a frequency channel over which the mobile stations can communicate directly with each other in a simplex manner. Generally, direct communication in the trunked system is carried out for providing communication privacy between two communicating parties and not for increasing communication throughput. Furthermore, the mobile stations in such a trunked system communicate using a one way communication path that does not support duplex communication.
Therefore, there exists a need for a TDMA communication system that efficiently allocates duplex channels for direct communication between the mobile stations.